This application pertains to few-moded optical waveguides with a refractive index (Bragg) grating, and to optical communication systems that comprise such waveguides.
Bragg gratings (also referred to as refractive index gratings) in optical waveguides are known. Conventionally such gratings couple a forward-propagating core-guided mode in single mode fiber to the backreflected core mode.
Mode conversion gratings are also known. See, for instance, U.S. Pat. Nos. 5,717,798 and 5,740,292. The latter discloses reflective gratings that inter alia couple light in the fundamental mode (LP01) to the LP11 mode.
Mode coupling gratings can find a variety of uses in optical waveguide systems. For instance, they can serve as wavelength routing filters in WDM networks.
However, it has been found that reflective grating mode converters that efficiently convert LP01 radiation to LP11 radiation frequently are difficult to manufacture, due to the spatial degeneracy of the LP11 mode. Whereas one of the LP11spatial modes typically can be converted to LP01 by a LP01 to LP11 mode converter, the other LP11 spatial mode typically can not be so converted, due to the interference of two nearby degenerate spatial modes. This spatial degree of freedom is difficult to control. Thus, it would be desirable to have available a mode converter which does not involve coupling to or from a spatially degenerate mode.
In particular, there is a need for a mode converter which couples the LP01 mode to the LP02 mode, without coupling the LP01 mode to any other guided mode, e.g., LP11 and reflected LP01. Such an LP01-LP02 mode converter could be used, for instance, in an add/drop multiplexer. This would avoid the need for an expensive and lossy circulator. Circulators are essential in implementing prior art Bragg grating-based add/drop filtering applications.
The LP02 mode is not spatially degenerate and thus can be efficiently converted to LP01. However, a LP01 to LP02 mode converting reflective grating typically must be designed such that both the LP01 to LP11 mode conversion and the LP01 to LP01 mode conversion are substantially nulled. The LP11 mode must exist in the optical fiber since, in order for the LP01 to LP02 mode conversion to be strong, the optical fiber must guide the LP02 mode, and therefore must also guide the LP11 mode. To the best of our knowledge, the prior art does not provide a technique for making such a reflective LP01 to LP02 mode converter. This application inter alia discloses such a mode converter.
C. X. Shi, IEEE Journal of Quantum Electronics, Vol. 32(8), August, 1996, page 1360, provides a theoretical treatment of a fiber-optic Fabry-Perot resonator with two mode conversion (LP01 to LP02) xe2x80x9cmirrorsxe2x80x9d. See also C. X. Shi et al., Optics Letters, Vol. 17(23), page 1655, December 1992; and F. Bilodeau et al., Electronics Letters, Vol. 27(8), page 682, April 1991.
M. J. Holmes et al., ECOC ""99, Sep. 26-30, 1999, Nice, France, pages I-216-217 disclose a fiber for sidetap filters. The fiber had a non-photosensitive core dopant for normalized radius less than 0.4, a combination of a non-photosensitive core dopant and germania for normalized radius 0.4-1, and a photosensitive cladding doped with germania out to a normalized radius of 3.5, to which boron was added to reduce the cladding index to match the deposition tube. The germania concentrations for the regions 0.4-1.0 and 1.0-3.5 were in the ratio 0.6:1.0 in order to obtain the required relative photosensitivity. The Holmes et al. paper thus discloses fiber in which the core had two different photosensitivity levels, with the cladding also being photosensitive. The photosensitivity profile was chosen to optimize the wavelength dependence of the cladding mode loss spectrum for applications, and not to obtain a mode converter of the herein relevant type.
All cited references are incorporated herein by reference.
For ease of exposition the discussion herein will generally refer to optical fibers. It will be appreciated, however, that similar results are obtainable in other optical waveguides, e.g., in planar waveguides.
The xe2x80x9ccoupling strengthxe2x80x9d between two guided core modes in a few-moded optical fiber is conventionally expressed in terms of an overlap integral, as shown in equation 2) below. The coupling strength typically depends on the refractive index profile n(r), the photosensitivity profile p(r), and the tilt angle xcex8.
xe2x80x9cMinimizingxe2x80x9d the coupling between two guided core modes in a given waveguide means adjusting the tilt angle of a tilted grating such that the coupling strength between the two modes is less than xe2x88x9230 dB.
xe2x80x9cMaximizingxe2x80x9d the coupling between two guided core modes in a given waveguide means adjusting the tilt angle of a tilted grating such that the coupling strength between the two modes is at least about xe2x88x9210 dB.
By a xe2x80x9cregular nullxe2x80x9d we mean herein a tilt angle region in a tilted (xe2x80x9cblazedxe2x80x9d) fiber Bragg grating that has a core mode coupling strength for light of a predetermined wavelength that is less than xe2x88x9230 dB over only a small (typically less than 0.1xc2x0) angular range of the tilt angle. Regular nulls occur for many tilt angles.
By a xe2x80x9csuper nullxe2x80x9d we mean two (or possibly more) regular nulls that occur at closely spaced tilt angles, thereby making the core mode coupling at the predetermined wavelength very low (typically less than xe2x88x9230 dB) over a relatively large (more than 0.1xc2x0, desirably more than 0.2xc2x0, or even 0.5xc2x0 or more) range of tilt angles between the regular nulls.
Modes of the guided light are designated LPmn in conventional fashion, with m and n being integers. Por instance, LP01 is the fundamental mode. LP01,f refers to the forward propagating fundamental mode, and LP01,b refers to the backward propagating fundamental mode.
xe2x80x9cPhotosensitivityxe2x80x9d refers to the refractive index change in the waveguide that results if an appropriately doped waveguide is exposed to actinic radiation, typically UV radiation.
A xe2x80x9cfew-modedxe2x80x9d optical waveguide supports the fundamental mode and one or more higher order modes, typically no more than about 10 guided modes total.
The description of the invention herein is generally in terms of conversion between the fundamental mode and a higher order mode such as LP02. This is for the sake of concreteness only, and the invention at least in principle can be embodied in an article for mode conversion between two appropriate higher order modes.
In an exemplary mode converter according to the invention, it is necessary that LP01,f light be strongly coupled into the LP02,b mode. However, in such a mode converter the coupling between LP01,f and all other guided reflected modes (exemplarily LP11,b and LP01,b) has to be small, exemplarily at least 20 dB less than LP01,f to LP02,b coupling. This simultaneous xe2x80x9cnullingxe2x80x9d of the coupling between LP01,f and the other guided modes (i.e., other than the desired coupling) can not be achieved with optical fiber that has uniform photosensitivity throughout the core, necessitating use of a more complex fiber design, as is described below.
The coupling strengths between the various guided modes in an optical fiber depend on the refractive index profile of the fiber and the electric fields of the various modes. Both of these parameters generally are fixed at the time of grating formation, and thus can not be varied to achieve a desired coupling. The only grating parameter which can be changed to significantly alter the relative coupling strengths is the tilt of the grating with respect to the core axis. However, with uniform photosensitivity in the fiber core, the control over the various coupling strengths that is achievable by introduction of a tilt in the grating is limited. In particular, with a uniform radial photosensitivity profile, it is impossible to xe2x80x9cnullxe2x80x9d simultaneously an even-even reflection (e.g., LP01,f to LP01,b) and an even-odd reflection (eg., LP01,f to LP11,b). Thus, we have determined that an additional degree of freedom has to be provided. This degree of freedom is the photosensitivity profile of the optical fiber. This profile has at least two distinct levels of photosensitivity in the core (of which one or more can be zero), and may, but need not, have substantially no photosensitivity in the cladding.
Thus, by way of example, forming a Bragg grating in an optical fiber wherein photosensitivity is removed (or substantially reduced) in certain regions of the fiber core makes it possible to achieve a much broader range of relative coupling strength as a function of the tilt angle of the fiber than is possible with fiber having uniform photosensitivity in the core. In particular, it is possible to simultaneously null both an even-even (e.g., LP01,f to LP01,b) and an even-odd (e.g., LP01,f to LP11,b) reflection, with strong LP01,f to LP02,b coupling, something that is not possible with a tilted refractive index grating that has uniform photosensitivity in the core.
More generally, the invention is embodied in an article that comprises an optical waveguide mode converter for converting light of wavelength xcex (exemplarily about 1.5 xcexcm) from a forward-propagating given guided mode to another predetermined guided mode. The mode converter comprises a tilted refractive index grating in the waveguide, the grating having a tilt angle xcex8 with respect to the waveguide axis and extending longitudinally over at least a portion of the waveguide. The waveguide is a few-moded waveguide for light of wavelength xcex and has a core and a cladding that contactingly surrounds the core. The fiber has a dopant distribution selected to provide the fiber with a refractive index profile n(r) and a photosensitivity profile p(r), with both profiles being functions of the radial coordinate r of the waveguide.
The mode converter has two or more non-zero coupling strengths among core guided modes, and p(r) has at least two different levels of photosensitivity in the core. Furthermore, n(r), p(r) and xcex8 are selected such that more than one of said non-zero coupling strengths are simultaneously nulled. For instance, n(r), p(r) and xcex8 are selected such that a given guided mode (e.g., LP01,f) is nulled with at least one other guided mode (e.g., LP01,b and LP11,b), and is strongly coupled to at least one other guided mode (e.g., LP02,b). In a preferred embodiment of the invention, the fundamental mode LP01,f is nulled simultaneously with an even and an odd backward-propagating guided mode (LP01,b, and LP11,b, respectively), and LP01,f is simultaneously strongly coupled to a higher order even mode (e.g., LP02,b).